1.1 · Data Types Beyond Module D

Goal: know structured and user-defined types — not just integer / float / string.

Recap from Module D

• Simple: integer, real (float), character, boolean.
• Structured: string, 1D array.

New for 2C

Structured data types

• 2D arrays (matrices, grids, game boards).
• Records / structs — grouped fields.
• Tuples — fixed-length immutable groups.
• Dictionaries / maps — key→value pairs.

User-defined types

Pythonic example:

class Student:
    def __init__(self, sid, name, score):
        self.sid = sid
        self.name = name
        self.score = score

alice = Student(1001, "Alice", 86)
print(alice.name, alice.score)

Or with dataclass (Python 3.7+):

from dataclasses import dataclass

@dataclass
class Student:
    sid: int
    name: str
    score: int

When to use each

Need	Type
Single value	simple type
Fixed-position values (rarely change shape)	tuple
Named fields	dataclass / dict
Many records of same shape	list of dataclasses / list of dicts
2D grid	list of lists

Worked example · Game board

ROWS, COLS = 3, 3
board = [[" "] * COLS for _ in range(ROWS)]
board[0][0] = "X"
board[1][1] = "O"
for row in board:
    print(row)

Common student mistakes

• Sharing inner lists by mistake: board = [[" "] * COLS] * ROWS creates rows that all share the same list reference.
• Mutating a tuple (impossible — they're immutable).
• Mixing list and dict access (d[0] vs d["key"]).

Exam-style question

Q (5 marks): Define a Python dataclass for a Book with title (str), author (str), price (float), year (int). Create two instances and print only books published after 2020.

Sample answer:

from dataclasses import dataclass

@dataclass
class Book:
    title: str
    author: str
    price: float
    year: int

books = [
    Book("HKDSE ICT Guide", "Lee", 99.0, 2024),
    Book("Vintage Algorithms", "Knuth", 120.0, 1973),
]

for b in books:
    if b.year > 2020:
        print(b.title)

Key takeaways

• Move beyond simple types when the data has structure.
• Prefer dataclasses / typed records over loose dicts.

➡️ Next: 1.2 Nested Loops & 2D Lists